A pilot evaluation of HER2/neu status between primary breast cancer and metastatic sites developed during trastuzumab-based therapy in patients with metastatic breast cancer (MBC)

2094 Background: The aim of this prospective study was to evaluate mechanisms of resistance to trastuzumab in patients with MBC. Methods: Sixteen patients with Her2/neu overexpressing tumors (15 were 3+ by immunohistochemistry (IHC) and one 2+ by IHC and positive by FISH) were eligible. Fourteen patients underwent biopsy and two fine needle aspirations (FNA) of newly developed lesions during transtuzumab treatment. All samples were analyzed by immunohistochemistry (IHC) (Herceptest). In all patients serial measurements of serum Her-2/neu (S-HER2) were made at baseline and during trastuzumab-based treatment by enzyme-linked immunosorbent assay (ELISA by Bender MedSystems) (cutoff point:10ng/ml). Patients whose HER-2 status changed in the metastatic site consisted group A, while the rest of the patients consisted group B. Results: The metastatic lesions in 6/16 patients (37.5%) (Group A) scored 0, +1. The rest of the cases 10/16(62.5%) (Group B) retained HER-2 overexpression in the metastatic site (+3). FNA...